1. Field of the Invention
The present invention relates to a zinc oxide piezoelectric crystal film provided on a sapphire plane, which is employed for propagating surface acoustic waves in a surface acoustic wave device.
2. Description of the Background Art
A typical surface acoustic wave (SAW) substrate for a SAW device is prepared by providing a piezoelectric crystal film on a non-piezoelectric substrate. In such a SAW substrate, a transducer which is formed by an interdigital electrode is provided on an outer surface of the piezoelectric crystal film or an interface between the piezoelectric crystal film and the non-piezoelectric substrate, in order to excite surface acoustic waves.
In relation to the aforementioned SAW substrate, known is a SAW substrate which is formed by a non-piezoelectric substrate of sapphire (.alpha.-Al.sub.2 O.sub.3) and a piezoelectric crystal film of zinc oxide (ZnO). In relation to such a zinc oxide/sapphire SAW substrate, further, (0112)-plane cut sapphire (hereinafter referred to as "R-plane sapphire") is employed so that (1120)-plane zinc oxide (hereinafter referred to as "Q-plane ZnO") is epitaxially grown on its R-plane.
FIG. 1 shows a SAW substrate 3 which is obtained by growing a Q-plane ZnO epitaxial film 2 on an R-plane sapphire substrate 1, as described above. In relation to such a SAW substrate 3, it is known that the SAW substrate 3 provides a high sound velocity and high coupling when the R-plane of the sapphire substrate 1 and the Q-plane of the ZnO epitaxial film 2 are parallel to each other and the [0001] direction (C-axis direction) of the ZnO epitaxial film 2 and the [0111] direction (D-axis direction) of the sapphire substrate 1 are in parallel with each other as shown by arrows in FIG. 1.
In order to form the ZnO epitaxial film 2 on the sapphire substrate 1, chemical transportation, CVD or sputtering is employed in general, while sputtering is widely employed among such processes, in particular.
When a pure ZnO thin film is formed on an R-plane sapphire substrate by sputtering, for example, its orientation is still insufficient for putting a SAW substrate which is formed by such a ZnO thin film into practice, although the C-axis of ZnO is parallel to the surface of the sapphire substrate to some extent and a certain degree of orientation is attained on the Q-plane of ZnO. As the result, characteristics of the interdigital transducer such as the electromechanical coupling factor are inferior to predetermined values in a SAW device employing such a SAW substrate.